Could CO(2)-induced land-cover feedbacks alter near-shore upwelling regimes?

Abstract

The response of marine and terrestrial environments to global changes in atmospheric carbon dioxide (CO(2)) concentrations will likely be governed by both responses to direct environmental forcing and responses to Earth-system feedbacks induced by that forcing. It has been proposed that anthropogenic greenhouse forcing will intensify coastal upwelling in eastern boundary current regions [Bakun, A. (1990) Science 247, 198-201]. Focusing on the California Current, we show that biophysical land-cover-atmosphere feedbacks induced by CO(2) radiative forcing enhance the radiative effects of CO(2) on land-sea thermal contrast, resulting in changes in eastern boundary current total seasonal upwelling and upwelling seasonality. Specifically, relative to CO(2) radiative forcing, land-cover-atmosphere feedbacks lead to a stronger increase in peak- and late-season near-shore upwelling in the northern limb of the California Current and a stronger decrease in peak- and late-season near-shore upwelling in the southern limb. Such changes will impact both marine and terrestrial communities [Bakun, A. (1990) Science 247, 198-201; Soto, C. G. (2001) Rev. Fish Biol. Fish. 11, 181-195; and Agostini, V. N. & Bakun, A. (2002) Fish. Oceanogr. 11, 129-142], and these and other Earth-system feedbacks should be expected to play a substantial role in shaping the response of eastern boundary current regions to CO(2) radiative forcing.

Fig. 1.

Simulated land–sea temperature contrast and anomalies. (A) Land–sea temperature contrast (calculated as land – ocean) for the CONTROL (solid line), 2XCO₂ (large dashed line), and 2XCO₂ + VEG (small dashed line) cases. (B) Anomalies in land–sea temperature contrast calculated as 2XCO₂ – CONTROL (solid line), 2XCO₂+VEG – 2XCO₂ (large dashed line), and 2XCO₂+VEG – CONTROL (small dashed line). A buffer of eight grid points was discarded from each side of the rectangular RCM domain. Land temperatures were calculated from all RCM land grid points within this buffer. Sea temperatures were calculated from all RCM ocean grid points within this buffer.

Fig. 2.

Simulated California Current wind-stress curl. (A) May curl in the 2XCO₂+VEG case. (B) August curl in the 2XCO₂+VEG case. (C) September curl in the 2XCO₂+VEG case. (D) May curl in the 2XCO₂ case. (E) August curl in the 2XCO₂ case. (F) September curl in the 2XCO₂ case. (G) May curl anomalies calculated as 2XCO₂ – CONTROL. (H) August curl anomalies calculated as 2XCO₂ – CONTROL. (I) September curl anomalies calculated as 2XCO₂ – CONTROL. Units are 10^–7 N/m³. Continental areas in the RCM are shown in white. Two coastlines are shown. The jagged line represents the RCM coastline. The smooth line represents the actual coastline. RCM grid boxes are 40 × 40 km.

Fig. 3.

Zonally averaged wind-stress curl anomalies for the California Current region. (A) Zonally averaged curl in the CONTROL case. (B) Zonally averaged curl in the 2XCO₂ case. (C) Zonally averaged curl in the 2XCO₂+VEG case. (D) Anomalies calculated as 2XCO₂ – CONTROL. (E) Anomalies calculated as 2XCO₂+VEG – 2XCO₂.(F) Anomalies calculated as 2XCO₂+VEG – CONTROL. Zonal averages were calculated over a band of six ocean points along the coast. Units are 10^–7 N/m³.

Fig. 4.

Regional response of the California Current to radiative forcing and land-cover–atmosphere feedbacks. Values of wind-stress curl in the CONTROL (small dashed line), 2XCO₂ (large dashed line), and 2XCO₂+VEG (solid line) cases were averaged over a band of six ocean grid points along the coast. (A) Northern limb (34°Nto42°N). (B) Southern limb (29.6°Nto34°N). Latitudinal definitions of the northern and southern limbs were based on the placement of the Southern California Bight (34°N) and Capo Blanco (42.5°N) on the RCM grid (see Fig. 2).